1. Field of the Invention
The present invention relates to a heat-treating apparatus for heat-treating, i.e. heating or cooling, substrates such as substantially circular semiconductor wafers.
2. Description of the Related Art
A substrate treating apparatus for effecting chemical treatment of substrates, such as resist application to or development of the substrates, performs heat treatment of the substrates as well. That is, the substrates are heated by a heating apparatus before and after the chemical treatment, and the heated substrates are cooled to room temperature by a cooling apparatus.
The heat-treating apparatus, i.e. the heating apparatus or cooling apparatus, for performing such heat treatment includes a heat-treating plate which may be a heating plate containing a heating device such as heaters, or a cooling plate containing a cooling device such as Peltier elements. The heat-treating apparatus heats or cools each substrate placed on the upper surface of the heat-treating plate.
The heat-treating apparatus described in U.S. Pat. No. 4,919,614, for example, includes three balls arranged on a heat-treating plate to protrude slightly from the upper surface thereof. A substrate is supported by the balls such that a minute spacing, called a proximity gap, is formed over the heat-treating plate between the upper surface of the heat-treating plate and the lower surface of the substrate. In this state, the substrate is heated or cooled.
Such heat-treating apparatus has support pins vertically movably extending through perforations formed in the heat-treating plate. In order to allow a substrate transported by a transport arm of a substrate transport mechanism to be placed on the heat-treating plate, the support pins are raised to position the upper, distal ends the support pins above the surface of the heat-treating plate. Then, the transport arm deposits the substrate on the support pins. Subsequently, the support pins are lowered to the substrate on the heat-treating plate. For transferring the substrate to the transport arm after heat treatment of the substrate on the heat-treating plate, has been completed the substrate support pins are raised to push the substrate up from the heat-treating plate. The substrate, in a raised position, is transferred to the transport arm.
The above heat-treating apparatus gives rise to a problem that the substrate may be placed horizontally immediately before the substrate is placed on the heat-treating plate or immediately after the substrate is raised from the heat-treating plate.
That is, when the support pins supporting the substrate are quickly lowered, the substrate is borne by air present between the lower surface of the substrate and the upper surface of the heat-treating plate since the two opposed surfaces are planar. The substrate may shift horizontally out of place under this supporting action of air. Such a phenomenon is amplified when the heated substrate is placed on a cooling plate. In this case, the substrate heats and expands the air between the lower surface of the substrate supported by the support pins and the upper surface of the cooling plate.
When the support pins are driven to raise the substrate having undergone heat treatment on the heat-treating plate, air does not flow smoothly into a space between the lower surface of the substrate and the upper surface of the heat-treating plate since the two opposed surfaces are planar. Consequently, the substrate supported by the support pins is subjected to a downwardly acting stress upon start of the ascent. As the substrate is raised in this state, a large quantity of air rapidly flows into the space between the lower surface of the substrate and the upper surface of the heat-treating plate. This cancels the stress applied to the substrate, and a reaction acts to lift the substrate above the support pins. As a result, the substrate may shift horizontally out of place.
Japanese Patent Publication (Unexamined) H4-56146 discloses a substrate treating apparatus having guide pins erected on a heat-treating plate for restricting a horizontal displacement of a substrate thereon.
However, the following problem is encountered where, as in the apparatus described in the above publication, guide pins are erected on a heat-treating plate. With short guide pins, a substrate may become misaligned in a position displaced from the guide pins. With tall guide pins, on the other hand, the apparatus must have a large construction to avoid the guide pins obstructing substrate transport. The tall guide pins have a further disadvantage of rubbing against edges of the substrate when the latter is raised or lowered, thereby producing particles.